It is a problem to determine whether a subject is impaired due to the use of alcohol or drugs, or whether the subject is suffering from sleep deprivation and/or a medical condition that could cause impairment. This impairment determination can be reliably accomplished by the measurement of eye gaze tracking and the monitoring of eye behavior (e.g., pupil size, blink rate, gaze, etc.) to determine a subject's alertness and attention.
For the detection of impairment, law enforcement officers in the field presently ask the subject to direct their gaze at a target which follows a predefined pattern defined by the Standardized Field Sobriety Test and/or Drug Recognition Expert protocols as established by the National Highway Traffic Safety Administration (NHTSA). By tracking the eye movements and measuring the pupil size and its responses, it is possible to determine a subject's impairment status. For instance, a subject who has been smoking marijuana demonstrates a very slow pupil reaction to abrupt changes in lighting conditions, where the pupil regains a dilated state even under bright light, sometimes called ‘rebound dilation’. While under the influence of heroin, the subject's pupil remains fully constricted, even in the dark i.e., pinpoint pupil. If a subject has been drinking alcohol, their eye movements become ‘jerky’ on movements to the right or left, i.e., ‘lack of smooth pursuit’. Upon lateral gaze, they can also exhibit a constant ‘jerk’, called horizontal gaze nystagmus (HGN). Therefore, the tracking of changes in the pupil's centroid location and/or size and reaction to light changes can be used to indicate impairment.
With all of these metrics, the accurate observation, tracking, and measurement of the subject's pupil centroid location and area is critical to the determination of impairment. It is also important in some applications to observe and detect eye blinking in order to compute the eye blinking rate to determine drowsiness and to ensure that pupil measurements are not computed at those time instances when a blink occurs, since the pupil is either partially visible or not visible at all when a blink occurs.
Presently, all of these measurements are typically performed manually with the accuracy and reliability of these measurements being subjective and possibly tainted by the ambient conditions under which the test is administered. Ambient conditions include the inability to maintain the subject's gaze in a fixed direction, erratic lighting conditions, and the subjective nature of the test administrator's administration of the test. Alternatively, standard methods of video taping of the eye responses for the Standardized Field Sobriety Test have been used, such as having one camera mounted on a tripod or handheld and aimed at the subject, but these applications can also be tainted by the ambient conditions under which the test is administered, further including camera aiming and maintaining a steady video image when the video camera is handheld.